1. Field of the Invention
The present invention relates to a spherical pigging device for passage through a pipeline which may be detected by magnetic sensors located external to the pipeline.
2. Prior Art
Pipeline "pigging" devices are used for cleaning, testing, gauging or operating a pipeline and are adapted to fit in sealable engagement with the interior of the pipeline. The pigging device may be moved through the pipeline in a number of ways. One way is to pressurize the pipeline with fluid behind the device to move it through the pipeline.
The need to detect passage of solid objects such as pigging devices moving through pipelines has been served by a number of instruments. A detection mechanism or signalling device senses movement of protrusions through the pipeline. One type of sensing device is physically attached to the pipeline through an opening and functions throughout the life of the pipeline. Usage may vary from every few hours to every few years. Periodic maintenance is required which requires special equipment operated by trained personnel that is performed on operating pipeline. If the signalling device is not maintained, it may leak the pipeline contents into the environment and impair the operability of the signalling device.
To alleviate the need for an opening to be cut into the pipeline and frequent maintenance, several types of non-intrusive signalling devices have been developed. One type of non-intrusive signalling device sends ultrasonic signals through the pipeline wall to sense the location of the pig. The ultrasonic devices are relatively expensive, require tight physical attachment to the pipe wall and have to be tuned with relation to the substance inside the pipeline.
Another type of non-intrusive signalling device senses the magnetic field change when a pig containing a magnet passes the signalling location. The pipeline wall is magnetically saturated with the magnetic field extending well outside the wall. A number of different devices of magnetic non-intrusive signalling devices have been produced. The magnetic non-intrusive signalling device is typically inexpensive, is easily mounted to the pipeline, is relatively small and is easily transportable. Additionally, the magnetic type of non-intrusive signalling device is relatively unaffected by the substance within the pipeline. The magnetic signalling device must, however, be used with a pig that has magnets on the pig.
The majority of pipeline pigs are composed of a number of cups forming a tubular body or a number of disks connected to a tubular body. A bar magnet is typically added to the tubular body with the poles of the magnet displaced axially along the axis of the pig body. The pole orientation of the magnet with respect to the axis of the pipeline is, therefore, maintained by the cups or the disks of the pig.
Another type of pipeline pig is a sphere which may be inflatable. Spherical pigs used in pipelines have a number of advantages. First, they are relatively easy to launch and receive. Second, the spherical pigs will even travel well during bends in the pipeline. Third, spherical pigs can be inflated to compensate for wear which occurs over time.
At the same time, however, a spherical pig is not restrained from rolling or rotating and moving in the pipeline. Accordingly, the magnetic field of the simple bar magnet may not reach the signalling device. Additionally, the magnetic pole orientation, with respect to the pipeline of a bar magnet added to the sphere, would not be constant or controllable once moving in the pipeline.
In addition to the proposal for the use of a single magnet on a sphere, plural magnets on a sphere have also been proposed. An example is shown in U.S. Pat. No. 3,546,642 (Frederick et al.), wherein a plurality of magnets are disposed on a magnetic sphere. Each of the magnets is an independent weak magnet having an independent magnetic field. No provision is made for coordinating the orientation of the magnetic fields.
There is a need, therefore, for a spherical pigging device having a plurality of magnets wherein the magnetic fields of the magnets are coordinated.
There is also a need to provide a process to produce a spherical pigging device wherein the magnets are embedded within the hollow body of the device and not subject to wear or breakage.